The present invention relates to the valve art generally and, in particular, certain embodiments of the present invention relate to precision metering valves of the type which are capable of metering minute amounts of fluid from a high pressure environment to a low pressure environment, e.g. from a high pressure into a hard vacuum.
One of the more serious problems encountered by those who are required to utilize valve devices has been the propensity of known valve devices to be damaged and rendered inaccurate or inoperable as a result of the valve being dropped or otherwise subjected to a shock loading. The damage experienced by the valves is most often to the valve operator, e.g. the handle or spindle, or to the valve stem, i.e. the member which operably connects the operator to the flow control structure such as a head and seat structure or a metering needle structure. If a valve is dropped and the handle is struck a jarring blow, the valve stem may be bent thus rendering the valve immediately inoperable, out of alignment or subject to wear which ultimately may cause valve failure. This problem of damage to the valve operating structure is particularly significant for highly sensitive valves such as precision metering valves. In such valves flow rate conditions often are predetermined and any damage to the operating mechanism may cause the valve to be carried out of calibration without the knowledge of the operator. Where flow rates and the precision metering of fluids are important to a process, test procedure or the like, and where a valve may be damaged without the knowledge of the operator, the effect of an improperly calibrated or operating valve may cause failure of the test and experiments, sometimes with attendant personal injury.
A further problem encountered by those in the valve art who deal with fine adjustment valves such as precision metering valves has been the difficulty experienced in positioning, properly, the flow controlling means associated with the valve for purposes of establishing a pre-selected flow rate in the circumstance of a known pressure differential. Such accurate positioning capability is required in many high precision metering valves, e.g. valves for controlling accurately the entrance of minutes quantities of fluid into a hard vacuum where the quantity of fluid admitted to the vacuum may be desired to be so small as to approach molecular quantities. The problem is rendered more complex where the fluid is being admitted to the vacuum from an evironment of much greater pressure, e.g. atmosphere or more.
With respect to these further problems, it has been proposed that a micrometer type guage be utilized in precision metering valves as the valve operator. Thus, the rotating portion of the guage is attached to the valve stem for rotation and displacement therewith and also to serve as the valve handle. As noted above, however, such an exposed valve handle gives rise to a high incidence of damage which is even more pronounced in the delicate mechanisms of such high precision valves. Further, the threaded connection between the stem and casing, and the fact that the casing serves as the base for the stem position indicator, introduces a thread-play error which is undesirable in highly accurate valves.
A still further problem which has been experienced with respect to high precision flow control valves relates to inaccuracies caused by the rotation of the flow control means during operation of the valve. More particularly, it is often the case that at least one element of a flow control means, e.g. a tapered needle, is mounted on an end of the valve stem for displacement into and out of flow permitting position. Because of the difficulties experienced in most precision machining methods, and because of the inherent eccentricity experienced in the tapered needles during manufacture and assmbly, the rotation of the valve stem and therewith the tapered needle during operation of the valve generates a saw-tooth flow output curve. Those skilled in this art will recogize immediately that such a saw-tooth flow output curve is undesirable and that a smooth flow output curve always is to be sought.
Yet another problem with known flow control valves is the cumulative effect of one-side tolerances during manufacture. More specifically, in the manufacture of a member having plural parts, it is ordinary practice to assume that individual inaccuracies, each within tolerance, will be above or below the design dimensions to effect a new cancelling effect thus placing the over-all final dimensions within a desired tolerance. Although such an assumption is valid in most instances, there are certain situations wherein the inaccuracies all occur as to be totally cumulative thus causing the finished product to be out of tolerance. Where such cumulative inaccuracies occur, the valves must be discarded as being outside specification thus resulting in reduced yields and increased manufacturing costs.
A still further difficulty with known valves is that the "wetted" area of the valves, i.e. the area within the valves which is exposed to the fluid being controlled, has been of such great relative size that virtually all the components of known valves have been required to be manufactured of materials which are compatible with the chemistry and pressure requirements of the fluid being valved. Often this becomes very expensive and is thus something to be avoided.